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Aerodynamic and thermal load interactions with lightweight advanced materials for high-speed flight

Final Report Summary - ATLLAS (Aerodynamic and thermal load interactions with lightweight advanced materials for high-speed flight)

The project ATLLAS aimed to evaluate and assess potential high-temperature resistant materials for sustained super- and hypersonic flight. The objective was to identify and assess lightweight advanced materials and structures along with more general technologies, including simulation tools, enabling to withstand ultra high temperatures and heat fluxes encountered during high-speed flight above Mach 3. At these high speeds, classical materials used for airframes and propulsion units are not longer feasible and need to be replaced by high-temperature, lightweight materials, with active cooling of some parts.

First, the overall design for high-speed transport vehicles was revisited to explore potential increase in lift / drag ratio and volumetric efficiency through the 'compression lift' and 'waverider' principles, taking into account sonic boom reduction. Second, materials and cooling techniques and their interaction with the aero-thermal loads were addressed for both the airframe and propulsion components. The former focused on sharp leading edges, intakes and skin materials coping with different aerothermal loads, the latter on combustion chamber liners. After material characterisation and shape definition at specific aerothermal loadings, dedicated on-ground experiments were conducted. Both ceramic matrix composites (CMC) and heat resistant metals were tested to evaluate their thermal and oxidiser resistance. In parallel, novel cooling techniques based on transpiration and electroaerodynamics principles were investigated. Combined aero-thermal experiments tested various materials specimens with a realistic shape such as sharp leading edges at extreme aero-thermal conditions for elevated flight Mach numbers. Dedicated combustion experiments on CMC combustion chambers allowed the reduction of combustion liner cooling and an overall thermal efficiency increase. The effect of NOx-production was assessed for these high-speed engines. Finally, particular aero-thermal-material interactions strongly influence the aerothermal loadings. Conjugate heat transfer, transpiration cooling and compressible transition phenomena were investigated and modelled.

Two aircraft configurations, suited to Mach 3 and Mach 6 flight were investigated. For both configurations, the major issues to be addressed from an aerodynamic point of view were sonic boom, aero-thermal loads, aerodynamic performance and fuels with good heat sink capability. The airplanes were being designed by two teams in parallel and integrated both almost by the same organisations, i.e. DLR, ESTEC, FOI, GDL, ONERA and UPMC. The selected design approach followed for each configuration has been completely different. The Mach 3 vehicle was the result of an inverse design strategy based on a method of characteristics where cruise efficiency and sonic boom mitigation have been the design variables. On the other side, the design of the Mach 6 vehicle was the result of applying a high-fidelity multi-disciplinary design optimisation technique to an existing vehicle configuration.

High-speed aircraft are exposed to high thermal and mechanical loads. Especially the nose structure, wing leading edges, air intakes etc. reaches temperatures which require special heat resistant materials, structural concepts and optional cooling devices. As the requested aerodynamic performance for an aircraft focuses on a high aerodynamic lift to drag ratio requiring sharp leading edges and rather thin wing or stabiliser structures. The utilisation and implementation of both of these requirements demands a detailed investigation of lightweight airframe materials including material design, coatings and determination of basic material properties withstanding high temperature requirements. This includes various tests on material samples to determine temperature stability, chemical resistance to the expected operational environment, mechanical stability and required additional physical properties. Three general classes of material investigation were considered: metallic hollow-sphere packings (HSP), ultra high temperature ceramics (UHTC) and ceramic matrix composites (CMC).

For the purposes of testing novel cooling concepts for increased engine thermal efficiency, activities were focused on increasing the combustion temperature by using lightweight materials and novel cooling concepts of the combustion chamber liners. Different fuels were taken into account, which are required for the studied aircraft concepts: kerosene and cryogenic fuels. Both high-pressure (aerojets) and ramjets based combustion chambers were used as a test bed to deal with realistic operational gas flow conditions. Different cooling techniques were investigated under a wide parameter range: film, effusion, transpiration and regenerative cooling. The thermal performance was investigated and a comparison to the simulation results was performed. A special work package was dedicated to the forecast of NOx emissions and the investigation of possible countermeasures to reduce these.

The emphasis in the area of loads definition is to develop and verify models to predict the combined effect of aero-thermal and material interaction on several lightweight high-temperature resistant materials. This is accomplished by integrating existing aerodynamic, heat-transfer, and structural codes. The results are then calibrated and verified with simplified experiments. The generic high speed aircrafts is a support for the requirements of both the coupled phenomenon computational tools and the experimental works: TUM for high pressure combustor for turbojets and ITLR for ramjet-based combustors related to cooling techniques or advanced materials investigation. The TUM combustion experiment was computed with different approaches. MBDA computed in 3D with CFD-ACE while Astrium did some 2D with its empirically tuned code ROCFLAM. Comparison made on heat fluxes on the different segments of the water-cooled calibration chamber gave correct agreement with experiment, except in the first segment, where mixing and ignition takes place. ESTEC and MBDA focused on the modelling of multi-physics flow phenomena within porous media. On the basis of the commonly-stated computational plan, several porous medium and combustion test cases have been computed with different levels of refinement. As the test cases are completed, the software porous media flow-through model implementations are validated versus the different experimental campaigns.

The study has shown that a cruise efficiency above three, i.e. L/D ratio times the propulsion efficiency, is recommendable for a long-haul cruiser. This can be achieved with the newly designed kerosene based Mach 3.5 vehicle M3T which is also well above the Concorde's figure of merit. With a 300 tons gross take-off weight, the 200 passenger vehicle achieves a range beyond 10 000 km (5 500 NM) after a 150 ton fuel burn. The hydrogen powered Mach 6 vehicle is however rather disappointing even after a dedicated optimisation process. With a GTOW of 278 tons including the 110 tons of hydrogen fuel, the 200 passenger vehicle's range could be brought up with 10 to 20 % to 7 400 km (4 000 NM) which is still below the envisaged 9 000 km. This doesn't mean a Mach 6 is intrinsically not conceivable, but indicates rather that a 'classical' design as proposed by Lockheed is not recommendable and should be avoided. A different architectural design or an improved engine design, including intake and nozzle, is needed to make it attractive. This perspective is not out of scope as a Mach 5 A2 vehicle conceived during the LAPCAT project can achieve this critical cruise efficiency. The better performance for the latter is mainly due to a well designed engine concept. During the project, significant steps were realised and promising results demonstrated. Work has nevertheless to be continued to increase the TRL of these technologies and take benefit of the existing test facilities and multi-physics engineering, industrial and dedicated simulation tools.